Sewage sludge thermal hydrolysis impact on the quality of biogas in anaerobic digestion process – siloxanes and VOCs studies

This paper presents the results of a study of biogas from the anaerobic digestion (AD) process of municipal sewage sludge (SS) under 43-48 o C temperature conditions, using the example of a wastewater treatment plant (WWTP) in Tarnów (Poland). In the case in question, the AD of SS is preceded by a 165 o C thermal hydrolysis process (THP) using Cambi THP® technology. The study of biogas concerned the basic parameters: temperature, relative humidity, content of CH 4 , CO 2 , O 2 , H 2 , N 2 , H 2 S, CO. Similar studies can be found in the literature, but they concern classical methane fermentation without considering the THP process. The present study comprehensively describes the relationship between the THP process and AD and the quality of biogas. The content of siloxanes and volatile organic compounds (VOCs) in biogas was also examined, including concentrations of Tetramethylsilane, Trimethylsilanol, L2, D3, L3, D4, L4, D5, D6, as well as toluene, benzene, xylenes and other VOCs. The results showed high concentrations of these compounds in raw biogas up to 42 mg/Nm 3 in total siloxanes and 15,8 mg/Nm 3 in total silicon, as well as the efficiency of biogas purification in an activated carbon filter with results of total siloxanes <0,31 mg/Nm 3 and total silicon <0,11 mg/Nm 3 . • The process of thermal hydrolysis and digestion of sewage sludge leads to the presence of organosilicon compounds and VOCs in biogas. • Basic biogas parameters i.e. temperature, relative humidity, content of CH 4 , CO 2 , O 2 , H 2 , N 2 , H 2 S, CO do not show any alarming values. • VOCs studies show a very wide range of compounds present in biogas. • Commonly used carbon bed filters do an excellent job of purifying biogas from organosilicon compounds.